Gasification processes are widely used to convert solid or liquid feedstocks such as coal, petroleum coke and petroleum residue into synthesis gas. Synthesis gas is predominantly composed of hydrogen gas (H2) and carbon monoxide (CO), and is utilized both as fuel for the production of electricity, as well as a feedstock for producing chemicals such as hydrogen, methanol, ammonia, synthetic/substitute natural gas or synthetic transportation oil. Synthesis gas produced via the gasification of carbonaceous material commonly contains some methane. The relative quantity of methane in the synthesis gas varies with the type of gasification system utilized, but is often observed to be higher in two-stage systems, such as ConocoPhillips E-Gas™ two-stage gasifier. Another example is the fixed-bed dry-bottom gasifier design of Lurgi GmbH (Frankfurt). In general, a significant amount of methane may be present in the syngas produced by any gasification system where the syngas leaves the reactor at a temperature of less than 2000° F.
The two-stage gasifier configuration has the benefit of a higher energy efficiency because a portion of the sensible heat in the hot synthesis gas leaving the first stage is utilized to gasify a portion of the feedstock added to the second stage in the absence of oxygen. Pyrolysis reactions dominate within the second stage and produce not just hydrogen and carbon monoxide, but also significant amounts of methane. Consequently, synthesis gas produced from a two-stage gasification reactor generally has a higher methane content than synthesis gas from most single-stage gasifier designs. For example, the synthesis gas produced in an E-Gas™ gasifier (ConocoPhillips Co.) usually contains between 1.5-4% methane (dry volume). This quantity of methane is not of significant concern when the synthesis gas produced is to be utilized as fuel for gas combustion turbines that generate electricity. However, this level of methane is not desirable when the synthesis gas is to be utilized as a feedstock for the production of chemicals, since H2 and CO are the components of synthesis gas utilized as feedstock for these chemical production processes, and in some instances, the presence of methane is detrimental to the intended chemical production process. An example of this is the production of butyraldehyde, where the process requires a feedstock synthesis gas containing less than 0.6% methane (by volume).
Current chemical production methods utilizing synthesis gas with a high methane content as feedstock commonly require separation of the methane from the raw synthesis gas. The resulting methane-rich purge gas is sometimes combusted as a fuel gas, or converted to additional hydrogen and carbon monoxide via steam-reforming of the methane at high temperature and pressure in the presence of a catalyst. Alternatively, the methane may be removed by other processes, such as cryogenic lean-oil absorption. However, these processes are expensive to build and operate. Accordingly, there exists a need for improved technology that allows production of raw synthesis gas with a decreased methane content by any gasification reactor wherein the synthesis gas normally leaves the reactor at a temperature less than 2000° F. The invention described herein provides a unique process for providing a low-methane synthesis gas as a feedstock for chemical production processes without the need for expensive pre-treatment to remove methane.